This appear to be the back of one of those 4x12's we see in old shots- possibly Lead Guitar Fill for Bob's side of the stage. Looks like three EV SRO's and a JBL D120. ... here is the front of the same cab:

Now here is evidence that, by 1970, the Fender Twins were already bereft of their speaker compliments! I think this is the rear of one of Bob's TR's.